Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Real estate investments at cost:
Notes receivable	$ 10,487	$ 10,480
Liabilities:
Notes payable	1,670,750	1,699,771
Unsecured credit facilities	164,550	164,514
Parent Company [Member]
Real estate investments at cost:
Land	1,435,619	1,432,468
Buildings and improvements	2,904,538	2,896,396
Properties in development	182,598	217,036
Gross real estate investments at cost	4,522,755	4,545,900
Less: accumulated depreciation	1,060,922	1,043,787
Total Cost Net of Accumulated Depreciation	3,461,833	3,502,113
Properties held for sale	26,861	0
Investments in real estate partnerships	287,500	306,206
Net real estate investments	3,776,194	3,808,319
Cash and cash equivalents	27,137	36,856
Restricted cash	3,893	3,767
Accounts receivable, net of allowance for doubtful accounts of $5,304 and $5,295 at March 31, 2016 and December 31, 2015, respectively	25,702	32,292
Straight-line rent receivable, net of reserve of $1,421 and $1,365 at March 31, 2016 and December 31, 2015, respectively	65,067	63,392
Notes receivable	10,487	10,480
Deferred Costs, Leasing, Net	69,200	66,367
Acquired lease intangible assets, less accumulated amortization of $47,470 and $45,639 at March 31, 2016 and December 31, 2015, respectively	112,513	105,380
Trading securities held in trust, at fair value	29,615	29,093
Other assets	27,565	26,935
Total assets	4,147,373	4,182,881
Liabilities:
Notes payable	1,670,750	1,699,771
Unsecured credit facilities	164,550	164,514
Accounts payable and other liabilities	157,732	164,515
Acquired lease intangible liabilities, less accumulated accretion of $18,603 and $17,555 at March 31, 2016 and December 31, 2015, respectively	43,751	42,034
Tenants’ security, escrow deposits and prepaid rent	28,217	29,427
Total liabilities	$ 2,065,000	$ 2,100,261
Commitments and contingencies (note 12)
Stockholders’ equity/Partners' capital:
Preferred stock, $0.01 par value per share, 30,000,000 shares authorized; 13,000,000 Series 6 and 7 shares issued and outstanding at March 31, 2016 and December 31, 2015, with liquidation preferences of $25 per share	$ 325,000	$ 325,000
Common stock, $0.01 par value per share,150,000,000 shares authorized; 97,610,485 and 97,212,638 shares issued at March 31, 2016 and December 31, 2015, respectively	976	972
Treasury stock at cost, 382,969 and 417,862 shares held at March 31, 2016 and December 31, 2015, respectively	(18,371)	(19,658)
Additional paid in capital	2,748,904	2,742,508
Accumulated other comprehensive loss	(72,893)	(58,693)
Distributions in excess of net income	(936,945)	(936,020)
Total stockholders’ equity	2,046,671	2,054,109
Noncontrolling interests:
Exchangeable operating partnership units, aggregate redemption value of $11,540 and $10,502 at March 31, 2016 and December 31, 2015, respectively	(1,989)	(1,975)
Limited partners’ interests in consolidated partnerships	37,691	30,486
Stockholders' Equity Attributable to Noncontrolling Interest	35,702	28,511
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,082,373	2,082,620
Total liabilities and equity	4,147,373	4,182,881
Partnership Interest [Member]
Real estate investments at cost:
Land	1,435,619	1,432,468
Buildings and improvements	2,904,538	2,896,396
Properties in development	182,598	217,036
Gross real estate investments at cost	4,522,755	4,545,900
Less: accumulated depreciation	1,060,922	1,043,787
Total Cost Net of Accumulated Depreciation	3,461,833	3,502,113
Properties held for sale	26,861	0
Investments in real estate partnerships	287,500	306,206
Net real estate investments	3,776,194	3,808,319
Cash and cash equivalents	27,137	36,856
Restricted cash	3,893	3,767
Accounts receivable, net of allowance for doubtful accounts of $5,304 and $5,295 at March 31, 2016 and December 31, 2015, respectively	25,702	32,292
Straight-line rent receivable, net of reserve of $1,421 and $1,365 at March 31, 2016 and December 31, 2015, respectively	65,067	63,392
Notes receivable	10,487	10,480
Deferred Costs, Leasing, Net	69,200	66,367
Acquired lease intangible assets, less accumulated amortization of $47,470 and $45,639 at March 31, 2016 and December 31, 2015, respectively	112,513	105,380
Trading securities held in trust, at fair value	29,615	29,093
Other assets	27,565	26,935
Total assets	4,147,373	4,182,881
Liabilities:
Notes payable	1,670,750	1,699,771
Unsecured credit facilities	164,550	164,514
Accounts payable and other liabilities	157,732	164,515
Acquired lease intangible liabilities, less accumulated accretion of $18,603 and $17,555 at March 31, 2016 and December 31, 2015, respectively	43,751	42,034
Tenants’ security, escrow deposits and prepaid rent	28,217	29,427
Total liabilities	$ 2,065,000	$ 2,100,261
Commitments and contingencies (note 12)
Stockholders’ equity/Partners' capital:
Preferred units of general partner, $0.01 par value per unit, 13,000,000 units issued and outstanding at March 31, 2016 and December 31, 2015, liquidation preference of $25 per unit	$ 325,000	$ 325,000
General partner; 97,610,485 and 97,212,638 units outstanding at March 31, 2016 and December 31, 2015, respectively	1,794,564	1,787,802
Limited partners; 154,170 units outstanding at March 31, 2016 and December 31, 2015	(1,989)	(1,975)
Accumulated other comprehensive loss	(72,893)	(58,693)
Total partners’ capital	2,044,682	2,052,134
Noncontrolling interests:
Limited partners’ interests in consolidated partnerships	37,691	30,486
Total noncontrolling interests	37,691	30,486
Total equity	2,082,373	2,082,620
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,082,373	2,082,620
Total liabilities and equity	4,147,373	4,182,881
Preferred Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	325,000
Common Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	976
Treasury Stock [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(18,371)
Additional Paid-in Capital [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,748,904
AOCI Attributable to Parent [Member]
Stockholders’ equity/Partners' capital:
Accumulated other comprehensive loss	(72,893)	(58,693)
AOCI Attributable to Parent [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(72,893)
AOCI Attributable to Parent [Member] | Partnership Interest [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(72,893)	(58,693)
Accumulated Distributions in Excess of Net Income [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(936,945)
Total Stockholders' Equity [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,046,671
Noncontrolling Interest Exchangeable Operating Partnership Units [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(1,989)
Noncontrolling Interests in Limited Partners Interest in Consolidated Partnerships [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	37,691
Noncontrolling Interests in Limited Partners Interest in Consolidated Partnerships [Member] | Partnership Interest [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	37,691	$ 30,486
Total Noncontrolling Interests [Member] | Parent Company [Member]
Noncontrolling interests:
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 35,702